Vernon Davis says he initially had to tell quarterback Colin Kaepernick to lighten up on his fastball in some cases, but the tight end adds: "You better be ready."

Photo: Stephen Lam, Special To The Chronicle

Vernon Davis says he initially had to tell quarterback Colin...

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Randy Moss' touchdown in New England showed Kaepernick's softer side, but another throw dislocated one of Moss' fingers.

Photo: Kelvin Ma, Special To The Chronicle

Randy Moss' touchdown in New England showed Kaepernick's softer...

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San Francisco 49ers wide receiver Randy Moss catches a pass for a touchdown against the Green Bay Packers during an NFL football game Sunday, Sept. 9, 2012, in Green Bay, Wis. By Matt Ludtke/SPECIAL TO THE CHRONICLE

Miller learned all about Colin Kaepernick's high-impact passes long before other 49ers receivers did. They worked out together at Competitive Edge Sports Performance, a training facility in Atlanta, before the 2011 NFL draft, and those sessions became Miller's introduction to the otherworldly strength of Kaepernick's right arm.

"It felt like every ball he threw was a 100-mile-an-hour hour fastball," Miller said last week. "My gloves were literally ripping and tearing. My fingers were jammed and calluses were torn up - through the gloves."

To listen to 49ers receivers, Kaepernick is the Justin Verlander of quarterbacks. Aaron Rodgers might throw with incomparable accuracy, but Rodgers - or virtually any other passer - cannot match Kaepernick for mind-blowing, finger-bending, glove-ripping velocity.

Miller and his colleagues point out that Kaepernick is learning to dial it down as needed. Now, after two years as a pro - including nearly a half-season as San Francisco's starter - he realizes the importance of occasionally putting some air under the ball, to make his passes more catchable.

Or, in baseball parlance: Sometimes, the situation calls for a changeup.

It made perfect sense to Miller when he heard Kaepernick really did throw 93-mph fastballs as a high school pitcher in Turlock (and was drafted by the Cubs). Miller, now the 49ers' fullback, played baseball in high school and could see the crossover.

But that was baseball and this is football. Kaepernick's predecessor, Alex Smith, is known more for accuracy than arm strength, so it took some time for 49ers receivers to adjust to the velocity of their new quarterback.

"Kaep has a very strong arm," tight end Vernon Davis said. "You better be ready for his passes, because if you're not he's going to hit you in the mouth.

"When he first got here, I had to tell him to slow down a little bit. It would be like a 5-yard route and he'd just drill it in there. And I'd have to say, 'Whoa, whoa, slow down.' He still has to work on it, but he's getting a lot better at that."

Randy Moss learned the hard way. Moss has caught passes from many strong-armed quarterbacks in his long career - Daunte Culpepper in Minnesota, Kerry Collins in Oakland, Tom Brady in New England - but none of those guys dislocated Moss' finger.

That's what happened Nov. 19 against the Bears, in Kaepernick's inaugural NFL start. He had a small window in which to fire a pass to Moss, so Kaepernick truly fired the ball.

Moss felt his finger pop when he tried to make the catch. He came back to the sideline, where a team doctor popped the finger back into place.

"I try to take pride in taking care of my body and finessing the balls as they come to me, but Kaep throws hard," Moss said later. "I tried not to show any tears, but my finger really did hurt."

Garrett Celek can sympathize. Celek, a rookie tight end from Michigan State, showed up for his first NFL training camp and dutifully started catching pre-practice passes from Kaepernick.

Kaepernick told Celek he appreciated the gesture, because Celek was the only receiver who didn't complain about how hard Kaepernick threw the ball.

"I have a lot more jammed fingers because of Kaep, that's for sure," Celek said. "He throws the ball so hard, at the beginning of practice if your hands aren't awake yet, they get all bruised up.

"You really have to focus in on the ball and catch it with your hands. If you're lazy, it's just going to go right through your fingers."

Celek didn't raise his hands quickly enough on one of Kaepernick's training-camp passes - and the ball bounced off Celek's face mask. That's not a good look, especially for a rookie trying to make the team.

All this good-natured banter aside, Kaepernick has showed an ability to change speeds. Take the game at New England on Dec. 16, when he lofted a soft pass to Moss for the 49ers' first touchdown and later rifled a knee-high laser beam to Michael Crabtree for the go-ahead score.

So, yes, it's still nice to have that extra gear.

Miller praised Kaepernick's progress in learning when to unleash his arm strength and when to harness it, saying 49ers receivers haven't dropped many of his passes. But Miller secretly longs for another challenge: digging into the batter's box.

Asked, playfully, if he would like to grab a bat and face Kaepernick, Miller didn't hesitate.

"I'm a big baseball guy, so I believe if he was on the mound I would hit him," Miller said.

Then he quickly caught himself and added, "But I faced a couple guys in high school who threw 97 or 98 mph, so I know what it looks like standing at the plate. It's not fun."